Desiccation cracking of earthen sites in archaeology museum – A viewpoint of chemical potential difference of water content
Desiccation cracking of earthen sites in archaeology museum – A viewpoint of chemical potential difference of water content
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DOI：
10.1177/1420326x15570810
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发表时间：
2015-02
影响因子：
3.6
通讯作者：
Xilian Luo;Z. Gu;C. Yu
中科院分区：
文献类型：
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作者：
Xilian Luo;Z. Gu;C. Yu
出土文物和土遗址是我们文化遗产的历史证据。其中许多被称为固定文物，因为文物本身及其周围都是我们的历史遗产，具有重要的历史意义，因此不能将其移入室内展示博物馆。研究人员研究了具体的环境规格，以建立室内展示博物馆的收藏品和游客所需的稳定温度，湿度和空气质量。然而，由于土遗址独特的展示特征，其保护仍然是环境科学家和技术人员面临的挑战，如何有效地保护这些古代遗址一直是全球关注的主要问题。考古博物馆正在开发先进的技术，以防止文物风化所造成的太阳，风和雨。由于对遗址保护认识不足以及室内小气候条件的影响，考古博物馆的原地文物远未得到良好的保护。由于文物的保存环境不合理，许多文物正在遭受破坏甚至毁坏。在出土文物所面临的危害中，干裂是最常见也是最棘手的问题。西安半坡博物馆土遗址的干燥层厚度大于0.4m（见图1（a））。秦始皇兵马俑博物馆也发现了同样的现象（见图1（B）），土遗址的含水量从1994年发掘时的15%~ 22%下降到现在的5%以下，遗址表面出现了严重的收缩和开裂。近年来，考古博物馆致力提升文物的保存水平，例如在文物保存制度中，保持较高的相对湿度和在展厅内进行空间划分。汉阳陵博物馆（中国西安）地下展厅就是这样一座现代考古博物馆（见图1（c））。它展示了第四位皇帝（公元前188-141年）的合葬墓中部分暴露的坑，其中包含陶俑。中国汉朝皇后文物领域的封闭空间在夏季保持在24摄氏度左右，冬季保持在16摄氏度左右。此外，相对湿度一直保持在95%以上的水平，以防止出土文物和遗址干燥。然而，展厅内的土遗址含水量仍从2006年发掘时的15%以上下降到2009年的4%左右。除干裂作用外，碳酸盐岩在大气-文物-表土复合界面上的扩散迁移机制也导致了石膏的存款。碳酸盐会与环境大气中渗透进来的二氧化硫发生反应，形成硫酸盐（见图1（d））。在高湿环境中发生干燥开裂的原因尚不完全清楚。保护协议，即空间划分和保持高相对湿度作为一种保护制度，已被考古学家认为是一种认可的做法，因此该协议适用于新的考古博物馆;然而，在兵马俑文物和土遗址中仍然存在裂缝。这些考古博物馆出土文物和出土遗址的干燥开裂似乎是一个自发的过程，即使大气环境的相对湿度接近上限值（100%）。为了阐明干燥开裂现象背后的水分迁移机制，验证高相对湿度是否足以防止出土文物的干燥开裂，
Unearthed relics and earthen sites are historical evidence of our cultural heritage. Many of them are referred to as immobile cultural relics since both the relics-in-itself and their surroundings are our historic heritage of past emperor’s burial with significant historical importance such that they cannot be moved into indoor-display museums. Researchers have studied the specific environmental specifications to establish the desired steady temperature, humidity and air quality for collections and visitors in indoor-display museums. However, due to the unique exhibition characteristics, the conservation of earthen sites is still a challenge to environmental scientists and technologists, and how to effectively protect these ancient sites has been a major concern globally. Archaeology museum is developing advanced techniques to prevent weathering of relics caused by the sun, wind and rain. The in-situ relics in archaeology museum are far from being well preserved because of the lack of understanding of site conservation and the impact of indoor micro-climatic conditions. Many of them are suffering deteriorations or even ruins due to improper preservation environment adopted for maintenance of relics. Among the hazards that unearthed relics are facing with, desiccation cracking is the most common and problematic. The drying layer of the earthen site in the Banpo museum (Xi’an, China) is thicker than 0.4m (see Figure 1(a)). The same phenomena are also found in the Emperor Qin’s Terra-Cotta Warriors and Horses Museum (see Figure 1(b)), the water content of the earthen site decreased from 15% to 22% in 1994 when it was excavated to less than 5% now, such that serous shrinkage and cracking occurred on the surface of the site. To upgrade the conservation of the relics in archaeology museums, great efforts have been made in recent years, e.g. maintenance of a high relative humidity (RH) and space division in the exhibiting hall have been implemented in archaeology museum as a part of the preservation regime. The underground exhibiting hall of Hanyangling Museum (Xi’an, China) is such a modern archaeology museum (see Figure 1(c)). It exhibits partially exposed pits containing pottery figurines in the co-burial grave of the fourth Emperor (188–141 B.C.) and Queen of Han Dynasty, China. The enclosed space for relics’ domain is being maintained at about 24 C in summer and 16 C in the winter. Moreover, the RH has been maintained at a level higher than 95% to prevent the drying of unearthed relics and sites. However, the water content of the earthen site in the exhibiting hall still decreased from over 15% when it was excavated in 2006 to around 4% in 2009. In addition to the desiccation cracking, gypsum was found to deposit on the surface of relics due to the transport mechanism by which the carbonates were diffused and migrated with moisture across the air-relic-topsoil complex interfaces. The carbonates would react with the penetrating SO2 from the ambient atmosphere to form sulphates (see Figure 1(d)). The reason why desiccation cracking occurs in high humidity environment is not entirely clear. The conservation protocol, i.e. space division and maintaining a high RH as a preservation regime, has been considered by archaeologists as an approved practice and therefore the protocol was applied to new archaeology museums; however, cracking still persists in the terracotta relics and earthen sites. The desiccation cracking of unearthed relics and excavated sites in these archaeology museums seems to be a spontaneous process even though the RH of the atmospheric environment was at approximately the ceiling value (100%). In order to clarify the mechanism of moisture migration behind the desiccation cracking phenomenon and to verify whether a high RH is adequate to prevent the desiccation cracking of unearthed in-situ relics, the chemical potential analysis